In wireline communication systems, the data is encoded on a modulated signal which is typically transmitted differentially over a pair of wires. In such systems, the common component of the differential signal on the two wires does not carry any information and it is typically noise from electro-magnetic interference (EMI) and crosstalk. Examples of wireline communication systems that are susceptible to common mode noise include various flavors of Digital Subscriber Line (DSL) and Ethernet over copper. In these systems the communication medium typically consists of multiple cascaded sections of twisted pair of wires typically terminated with transformers at both ends.
FIG. 1 shows a circuit 100 that includes a termination of a twisted pair of wires 110 into a transformer 130. The transformer 130 is coupled to a transceiver 140 and is primarily used for DC isolation and common mode rejection. FIG. 1 also shows a common mode choke 120 which is sometimes included to further block the common mode signal. It also provides proper impedance termination for common mode signal which prevents reflections and standing waves.
In a perfect system, the common mode component is very well balanced and does not convert to a differential signal. However, in any practical system the transversal conversion loss (TCL) is finite which means some portion of the common mode signal shows up as differential noise at the receiver. The imbalance that would cause a finite TCL can be anywhere in the signal path including the wireline channel, the connectors at both end of the channel, and even the components that is used to detect the differential signal. This noise may be a limiting factor in the performance of a wireline communication system. Therefore, it is important to be able to detect the common mode component for characterization purposes as well as potential improvement in the performance. The present invention addresses such a need.